1. Field of the Invention
The present invention generally relates to a method of forming a coupling structure of resonators, and more particularly, to a method of forming a coupling structure of non-adjacent resonators.
2. Description of Related Art
In a wireless communication system, frequency selection elements, such as filters, duplexers, multiplexers and so on, are necessary key elements for radio-frequency front-end circuits. The frequency selection elements have functions for selecting or filtering/attenuating signals or noise in a specific frequency range in a frequency domain, so that rear-end circuits can receive signals in a correct frequency range and process the signals.
In the frequency ranges of microwave (1 GHz-40 GHz) and millimeter wave (40 GHz-300 GHz), the entire radio-frequency front-end circuits are formed of waveguide tubes in a large system. A waveguide tube has advantages of high-power endurance and extremely low loss, but its minimal size is limited because of its cut-off frequency. In addition, the waveguide tube is manufactured in non-batch method by precision work, and thus, high cost limits the application coverage of the waveguide tube.
Japanese Patent Application Laid-Open Publication No. 06-053711 provides a high-frequency signal transmission structure of an equivalent waveguide tube, which is formed by a circuit board structure. As shown in FIG. 1, the structure is called as a substrate integrated waveguide (SIW), whose basic structure comprises a dielectric layer 3 and conductor layers 1 and 2. The SIW structure has advantages of low-cost and integration with plane circuits since the SIW can be implemented by using a general circuit board or other multilayered plane structures, such as low temperature cofired ceramic (LTCC), thereby. However, the SIW is formed of a multilayer board, thus its thickness is limited. Generally, the thickness is about several tens of mils, and the width is generally several hundreds of mils or more due to the restriction of the cut-off frequency (waveguide tube) or the resonant frequency (resonant cavity). The width-high ratio is usually greater than 10, and the width-high ratio for a conventional hollow waveguide tube is about 2. In comparison with the conventional waveguide tube, SIW with high width-high ratio may result in the following results. First, a flatter structure may result in higher metal-loss in the same width and the same transmission frequency, and therefore, the quality factor of the resonator is restricted. Second, in a flat structure, a number of resonators can be arranged in a vertical stack mode that occupies less area, so that the flat structure has advantages of small volume and high performance.
The coupling manner of a multi-stage resonator filter is related to the resonant mode and relative positions of the resonators. Nowadays, the staggered coupling manner using the SIW structure is to use a plane linear arrangement structure with an additional coupling mechanism, which is as shown in FIG. 2 (referring to X. Chen, W. Hong, T. Cui, Z. Hao and K. Wu, “Substrate integrated waveguide elliptic filter with transmission line inserted inverter”, Electronics Letter, Vol. 41, issue 15, 21 Jul. 2005, pp. 851-852), a plane U-shape arrangement shown in FIG. 3 (referring to Sheng Zhang, Zhi Yuan Yu and Can Li, “Elliptic function filter designed in LTCC”, Microwave Conference Proceedings, 2005. APMC. Asia-Pacific Conference Proceedings, Vol 1, 4-7 Dec. 2005) or a vertical U-shape arrangement shown in FIG. 4 (referring to Zhang Cheng Hao; Wei Hong; Xiao Ping Chen; Ji Xin Chen; Ke Wu; Tie Jun Cui, “Multilayered substrate integrated waveguide (MSIW) elliptic filter”, IEEE Microwave and Wireless Components Letters, Vol. 15, Issue 2, Feb. 2005 Page(s): 95-97). For SIW structure, resonators in a linear arrangement are not efficient, and the additional coupling mechanism is too long which is disadvantageous for the multi-stage filter. Regardless of plane or vertical bending structure for the U-shape arrangement and taking a filter with four resonators as an example, the first resonator should be adjacent to the fourth resonator in order to form the staggered coupling structure. As a result, such structure limits the flexibility of arrangement of the input/output port, and occupies more areas.
In summary, in conventional techniques, there is no any technique that provides a vertically-staggered coupling structure for non-adjacent resonators. The conventional techniques limit the flexibility of the input/output port, and occupy more areas.
Additionally, for the design of current filters, a transmission zero (TZ) is formed by using the coupling between non-adjacent resonators in a main coupling path (that is, a staggered coupling). When the TZ is set at a proper frequency, a larger amount of signal attenuation can be obtained; that is, the same attenuation amount can be obtained by using fewer stages, so that the pass-band loss is lower, and the volume is smaller. However, as described above, there is no design to efficiently form coupling between non-adjacent resonators. Thus, it is necessary for those of skill in the art to provide an efficient staggered coupling structure for non-adjacent resonators.